β-Lactam Susceptibility of Streptococcus dysgalactiae subsp. equisimilis

All clinical isolates of Streptococcus dysgalactiae subsp. equisimilis (SDSE) are considered susceptible to β-lactams, the first-line drugs used to treat SDSE infections. However, given that penicillin-non-susceptible SDSE strains have been isolated in Denmark, in this study, we aimed to identify β-lactam-non-susceptible clinical isolates of SDSE in Japan. In 2018, we collected 150 clinical isolates of S. dysgalactiae, and species identification was performed using a Rapid ID Strep API kit. The minimum inhibitory concentrations (MIC) of six β-lactams (penicillin G, oxacillin, ceftizoxime, ceftibuten, cefoxitin, and cefaclor) were determined for the 85 clinical isolates identified as SDSE using the agar dilution method standardized by the Clinical & Laboratory Standards Institute. The MIC ranges of penicillin G, oxacillin, ceftizoxime, ceftibuten, cefoxitin, and cefaclor were 0.007-0.06, 0.03-0.12, 0.015-0.06, 0.25-2, 0.12-2, and 0.06-0.5 μg/mL, respectively. None of the clinical isolates of SDSE were non-susceptible to penicillin G, indicating that all 85 clinical isolates of SDSE were susceptible to β-lactams. Our findings indicate that almost all clinical isolates of SDSE, from several prefectures of Japan, are still susceptible to β-lactams. Nevertheless, there remains a need for continuous and careful monitoring of drug susceptibility among clinical SDSE isolates in Japan.